The objective of this project is to test whether arc-continent collision events modify overall convergence rates, with the island of Timor as test-case. The mountainous island of Timor exposes a tectono-stratigraphic section of a typical arc-continent collision, with elements from the overriding Asian oceanic/arc plate, a wedge of high pressure/low temperature metamorphic rocks and an underlying deformed Australian continental passive margin sedimentary sequence. An accurate geologic map of the Island, which describes the distribution of rock types and structures, will be produced that will form the basis for balanced structural cross-sections, which, in turn, will be used to quantify how much the sedimentary rocks once deposited on the margin have been telescoped during collision. The magnitude of shortening should correlate with the amount of continental crust that has been subducted. Paleomagnetic analysis is used to determine paleolatitude, rotation, and age of strata. High- to low-temperature thermochronologic data will document the magnitude, variability, and timing of exhumation.
Shortly after the recognition of plate tectonics, the Wilson cycle was introduced in order to describe the creation and demise of ocean basins. The original four stages ? continental rifting, seafloor spreading and formation of ocean basins, closure of ocean basins by subduction of oceanic lithosphere, and continent-continent collision - still form a fundamental basis for our understanding of tectonic processes. Arc-continent collision marks the ultimate demise of an intra-oceanic subduction zone and is a common phenomenon during ocean closure. It is usually assumed that significant underthrusting of continental material under an oceanic island arc is inhibited by buoyancy but the magnitude of continental subduction remains unconstrained. This project will provide an estimate for the magnitude of continental crust that has been subducted, results that would have important implications for mantle chemistry and dynamics. The project involves a significant collaboration with researchers from Norway, Swiss, Australia, and Indonesia as well as training of U.S. and international students.
The transition from subduction to collision in the Sunda to Banda arcs, respectively, is documented by the following: 1) isostatic uplift of the Sunda Trench, 2) increased coupling of the oceanic upper plate to the continental lower plate causing indentation of, and distribution of strain away from, the thrust front, 3) progressive uplift and widening of the accretionary wedge, 4) progressive closure and subduction of forearc basement and 5) progressive continental contamination, decreased activity and uplift of the volcanic arc. We investigated these characteristic features of the active Banda arc-continent collision by employing a diverse methods that track the distribution of strain over a wide range of temporal scales. The methods include structural mapping of the Banda orogenic wedge, constraining the ages and structure of pre- and syn-orogenic rock units, analysis of protoliths and thermo-chronologies of metamorphic rocks, balancing cross-sections across various parts of the orogeny that represent different stages of development, studies of uplifted coral terraces and other geomorphic features, distributions of seismicity and geodetic measurements. The results of these studies challenge many of the current ideas about how arc-continent collisions evolve. For example: 1) continents can subduct to great depth and not experience slab tear or subduction polarity reversal; 2) balanced cross sections in East Timor reveal 326-362 km of shortening and that 215-229 km of Australian continental lithosphere has been subducted; 3) forearc basement can be pulled down with the subducting continental slab leaving very little trace in the geological record; 4) diapirism of shale and metamorphic rocks can be an important process in building an orogenic wedge; 5) the pattern of uplift revealed by warping of flights of coral terraces indicate short wavelengths associated with crustal deformation not the long wavelengths expected from lithospheric delamination or slab tear; and 6) the vergence directions of folds and thrust faults can differ by at least 90 degrees from plate convergence directions due to boundary conditions imposed by the subducting plate.
